1. Field of the Invention
The present invention relates to a magnetic head driving device and a magnetooptical recording apparatus incorporating the device.
2. Related Background Art
As a conventional recording method for a magnetooptical recording apparatus, an optical modulation method, a magnetic field modulation method, and the like are known. In particular, the magnetic field modulation method capable of directly overwriting new data on old data is superior to other methods in terms of, e.g., the recording speed. FIG. 1 shows a schematic arrangement of a magnetooptical recording apparatus adopting the magnetic field modulation method. In FIG. 1, a magnetooptical disk 1 as an information recording medium has a magnetooptical recording layer 1a. A magnetic head 2 prepared by winding an excitation coil C.sub.h around a magnetic core is arranged above the upper surface of the magnetooptical disk 1, and an optical head 4 is arranged below the lower surface of the disk to oppose the magnetic head 2. The optical head 4 radiates a laser beam emitted from a semiconductor laser arranged therein as a light source onto the recording layer la as a small beam spot so as to increase the temperature of a recording portion to a temperature equal to or higher than a Curie temperature of this portion. On the other hand, the magnetic head 2 is driven by a driving circuit 3 to generate a bias magnetic field modulated according to recording information, and applies the bias magnetic field to the temperature-increased portion of the recording layer 1a. Thus, the direction of magnetization of the temperature-increased portion on the recording layer 1a is oriented in the direction of the bias magnetic field, and an information pit is recorded on the recording layer 1a. Recently, in order to attain a higher information recording density, a pit recording method tends to transition from a pit position recording method for forming a significant information portion at the central position of a pit to a pit edge recording method for forming a significant information portion at the edge position of a pit. In the pit edge recording method, the edge of an information pit must be clearly recorded. For this purpose, it is required to increase the inversion speed of the bias magnetic field generated by the magnetic head in the recording mode.
As a driving device for the magnetic head, which can satisfy the above requirement, a device disclosed in, e.g., Japanese Laid-Open Patent Application No. 63-94406 is known. FIG. 2 is a circuit diagram showing the driving device. The device shown in FIG. 2 includes an excitation coil C.sub.h for generating the bias field of the magnetic head 2, and auxiliary coils L1 and L2 for switching the magnetic field at high speed. The device also includes switch elements S1 and S2 for switching the direction of a current flowing through the excitation coil C.sub.h, and resistors R1 and R2 for limiting current. The inductances of the auxiliary coils L1 and L2 are set to be sufficiently larger than that of the excitation coil C.sub.h. In this driving device, when the switch elements S1 and S2 are controlled to be alternately turned on so as to switch the direction of a current flowing through the excitation coil C.sub.h, the polarity of the generated magnetic field is switched according to recording information. More specifically, when the switch element S1 is ON, and the switch element S2 is OFF, current paths CH1 and CH4 are conducted, and current paths CH2 and CH3 indicated by broken lines are interrupted. At this time, since a current is supplied to the excitation coil C.sub.h upon conduction of the current path CH1, the coil C.sub.h generates a magnetic field corresponding to the direction of the current. On the other hand, when the switch element S1 is OFF, and the switch element S2 is ON, the current paths CH2 and CH3 are conducted, and the current paths CH1 and CH4 are interrupted. As a result, a current in a direction opposite to that described above is supplied to the excitation coil C.sub.h upon conduction of the current path CH2, and the coil C.sub.h generates a magnetic field having an inverted polarity. Since the inductances of the auxiliary coils L1 and L2 are sufficiently larger than that of the excitation coil C.sub.h, although the current paths are switched from CH1 to CH3 and from CH4 to CH2 before and after the ON/OFF operations of the switch elements S1 and S2, flowing currents maintain an almost constant value. For this reason, when the ON/OFF times of the switch elements S1 and S2 are sufficiently shortened, the direction of a current flowing through the excitation coil C.sub.h can be inverted in a very short period of time without increasing a voltage of a DC power supply V.
However, in practice, even when the switch element is turned off, if the switch element comprises, e.g., a field effect transistor, since a stray capacitance is present in its drain-source path, a vibration phenomenon is caused by this stray capacitance component and the inductance component of the excitation coil C.sub.h. Thus, the current inversion time of the excitation coil C.sub.h is determined by the period of the vibration. Such a current vibration phenomenon is gradually attenuated since its energy is consumed in the process of vibration. In order to attenuate the current vibration more quickly, a damping resistor R.sub.d is normally connected in parallel with the excitation coil C.sub.h, as shown in FIG. 2.
In the conventional magnetic head driving device shown in FIG. 2, the current inversion time of the excitation coil is undesirably determined by the vibration period defined by the stray capacitance around the switch element, and the inductance of the excitation coil. For this reason, in order to further shorten the current inversion time, the stray capacitance around the switch element and the inductance of the excitation coil must be decreased. However, when the switch element comprises a field effect transistor, the smaller the drain-source path capacitance becomes, the smaller the maximum rated current of the drain current becomes. Therefore, a decrease in stray capacitance is limited by the current value of the excitation coil. When the inductance is decreased by decreasing the number of turns of the excitation coil, a sufficient magnetic field strength cannot be obtained, and a decrease in inductance is also limited. In this manner, in the conventional device, it is difficult to further shorten the current inversion time of the excitation coil by merely decreasing the stray capacitance and the inductance.